Typically, a Metal Oxide Semiconductor (MOS) transistor includes a semiconductor substrate, a source, a drain, and a channel positioned between the source and drain on the substrate, which is usually made from silicon. Normally, a gate stack is located above the channel, the gate stack being composed of a gate oxide layer or gate electrode located directly above the channel, a gate conductor material above the gate oxide layer or gate dielectric, and sidewall spacers. The sidewall spacers protect the sidewalls of the gate conductor. The gate electrode is generally formed of doped polysilicon (Si) while the gate dielectric material may comprise a thin layer (e.g., <20 Å) of a high dielectric constant material (e.g., a dielectric constant greater than 4.0) such as silicon dioxide (SiO2) or nitrogen-doped silicon dioxide, and the like.
The amount of current that flows through the channel of a MOS transistor is directly proportional to a mobility of carriers in the channel, and the use of high mobility MOS transistors enables more current to flow and consequently faster circuit performance. Mobility of the carriers in the channel of an MOS transistor can be increased by producing a mechanical stress in the channel. A channel under compressive strain, for example, a silicon-germanium channel layer grown on silicon, has significantly enhanced hole mobility to provide a pMOS transistor. A channel under tensile strain, for example, a thin silicon channel layer grown on relaxed silicon-germanium, achieves significantly enhanced electron mobility to provide an nMOS transistor.
An nMOS transistor channel under tensile strain can also be provided by forming one or more carbon-doped silicon epitaxial layers. The carbon-doped silicon epitaxial layers can be deposited on the source/drain of nMOS and pMOS transistors, respectively. The source and drain areas can be either flat or recessed. When properly fabricated, nMOS sources and drains covered with carbon-doped silicon epitaxy imposes tensile stress in the channel and increases nMOS drive current.
To achieve enhanced electron mobility in the channel of nMOS transistors having a recessed source/drain using carbon-doped silicon epitaxy, typical processes include selectively forming the carbon-doped silicon epitaxial layer on the source/drain. In such processes, it is desirable for the carbon-doped silicon epitaxial layer to contain substitutional C atoms to induce tensile strain in the channel. Higher channel tensile strain can be achieved with increased substitutional C content in a carbon-doped silicon source and drain. However, most of C atoms incorporated through typical selective Si:C epitaxy processes (for example at process temperature >700° C.) occupy non-substitutional (i.e. interstitial) sites of the Si lattice. By lowering growth temperature, a higher fraction of substitutional carbon level can be achieved (e.g. nearly 100% at growth temperature of 550° C.), however, the slow growth rate at these lower temperatures is undesirable for device applications, and such selective processing might not be possible at the lower temperatures. Further, such processes encounter difficulty achieving desired selectivity.
Therefore, there is a need to provide methods to improve the substitutional carbon content in carbon-doped silicon epitaxial layers and the growth rate and selectivity of forming such layers.